In telephony systems, a foreign exchange station (FXS) may use SLICs to provide service to analog telephones. The SLICs are used to perform a number of different tasks such as supplying power to a telephone circuit, detecting telephone status (e.g., on-hook/off-hook), detecting dialing information, sending and receiving voice signals, supporting analog-to-digital and digital-to-analog conversion of voice signals, and other tasks.
Each telephone is connected to a SLIC by two wires that are referred to as Tip and Ring for legacy reasons. The telephony industry uses a negative voltage framework (i.e., a negative operating voltage on Tip and Ring with respect to earth ground) to prevent corrosion of the metal wires.
Many SLIC implementations employ a tracking voltage power supply that dynamically adapts its output voltage according to its present operating state and telephony loop characteristics. This allows it to generate only required voltages, thereby minimizing thermal dissipation and maximizing power efficiency.
Tip and Ring telephony wiring is subject to foreign electromagnetic disturbances that may be caused by a number of events such as lightning, proximity to other electronic wiring and/or devices (e.g., welding equipment), electrostatic discharge, and the like. The SLICs must be protected from the electromagnetic disturbances that exceed voltage ratings.
Overvoltage protectors may be used to protect SLICs from overvoltage disturbances imparted to the Tip and Ring wiring. There are two main types of overvoltage protectors commonly used with SLICs. One is a fixed overvoltage protector that limits voltages on the Tip and Ring between ground and a fixed negative potential. The other is a programmable overvoltage protector that limits the Tip and Ring voltages between ground and an externally-supplied, adjustable, negative voltage reference.
Programmable overvoltage protectors are often used with tracking power supply SLICs. Their programmable threshold is set according to the power supply output voltage. This provides improved margin against a maximum voltage rating for certain SLIC states. For example, on-hook and off-hook states demand lower supply voltages compared with a ringing state, whereas the ringing state usually requires negative voltages approaching the maximum rating of the SLIC. Thus, programmable overvoltage protectors having flexible ranges are often preferred over fixed overvoltage protectors.
Overvoltage protectors typically employ Thyristors, also known as silicon controlled rectifiers (SCRs), as controllable switches. In SLIC circuits, a Thyristor anode connects to circuit ground and a Thyristor cathode connects to the Tip (or Ring). For the programmable overvoltage protector, the Thyristor gate typically connects to the SLIC power supply voltage. The SLIC power supply voltage is normally more negative than that of the Tip or Ring, and thus the Thyristor gate-to-cathode junction is reverse biased, keeping the Thyristor in a non-conductive state. Should the Tip (or Ring) voltage become more negative than that of the SLIC power supply, the Thyristor gate-to-cathode junction becomes forward biased and gate conduction occurs. Sufficient gate current triggers the Thyristor to turn on, creating a low impedance path cathode to anode, effectively shunting the Tip (or Ring) to ground and protecting the SLIC. The programmable overvoltage protector device may be packaged as a pair of Thyristors, having a common control voltage pin for the SLIC power supply voltage connection, a common ground pin for connection to the anodes, and independent cathode pins for connection to the Tip and Ring conductors.
FIG. 1 depicts a commonly used topology of a SLIC circuit 110, an external tracking power supply circuit 120, and a programmable Thyristor-based overvoltage protection device 150. The SLIC control output 115 sets its negative output voltage level, conducting through steering diode 190, low pass filtered via capacitor 180, continuing to the SLIC 110 power input via diode 170. Voltage rail 160 connects to a gate of the overvoltage protection device 150. Should a negative voltage be imparted to Tip 130 or Ring 140 that is more negative than that of the voltage rail 160, Thyristor gate conduction occurs. Conduction current of sufficient magnitude into the gate will trigger the Thyristor. Thyristor gate conduction current is sourced by capacitor 180, and tends to charge the capacitor 180 more negatively with respect to ground. Capacitor current is determined according to the equation C*dv/dt. The dv/dt in this case is the rate of change of the transient voltage imparted to the Tip 130 or Ring 140 that is more negative than the voltage rail 160. With fast, negative-going transients, the gate current is sufficiently large to trigger the Thyristor-based overvoltage protection device 150 very quickly, protecting the SLIC. However, if dv/dt is relatively small, as might occur from a negative-going 50-60 Hz AC power line crossing the Tip 130 and/or Ring 140, the gate current may be insufficient to trigger the Thyristor. In this case the capacitor 180 will continuously charge negatively, causing the rail 160 to exceed a rating of the SLIC 110, resulting in electrical overstress damage.
Therefore, with slow changing, negative-going transient events, the output of the tracking power supply 120 may follow the imparted voltage, and the overvoltage protector 150 may fail to trigger the protection. Improvements are desired to provide an alternate source of Thyristor gate current should the voltage rail 160 exceed a negative threshold level.